ANDROID: fix KCFLAGS override by __ANDROID_COMMON_KERNEL__

Our test build is broken by KCFLAGS overrided in build.config.comm.

Since Linux Makefile supports 'export KCFLAGS=XXX' to customize the
KCFLAGS, and we should keep this functionality.

Bug: 230818006
Fixes: d2ed4cfcd5a5 ("ANDROID: Add flag to indicate compiling against ACK")
Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Change-Id: I9425d79697bc1fe816ce82d523f91631dee6b8f4
diff --git a/build.config.common b/build.config.common
index e981c0e..f671676 100644
--- a/build.config.common
+++ b/build.config.common
@@ -9,7 +9,7 @@
 CLANG_PREBUILT_BIN=prebuilts-master/clang/host/linux-x86/clang-r416183b/bin
 BUILDTOOLS_PREBUILT_BIN=build/build-tools/path/linux-x86
 
-KCFLAGS=-D__ANDROID_COMMON_KERNEL__
+KCFLAGS="${KCFLAGS} -D__ANDROID_COMMON_KERNEL__"
 EXTRA_CMDS=''
 STOP_SHIP_TRACEPRINTK=1
 IN_KERNEL_MODULES=1